Power electronic devices need to switch between ON and OFF of power supply to drive loads of, for example, electric motors. Thus, switching elements made of silicon such as insulated gate bipolar transistors (IGBTs) or metal-oxide-semiconductor field-effect transistors (MOSFETs) are used.
Switching devices intended to be used as power semiconductor devices often adopt the MOSFETs or the IGBTs with vertical structures (vertical MOSFETs or vertical IGBTs). Examples of the vertical MOSFETs include planar MOSFETs and trench (trench-gate) MOSFETs that are selected depending on the gate structure (for example, see Patent Document 1).
In the trench-gate MOSFETs in which gate trenches that are channels are formed in a drift layer of a first conductivity type (n-type) in a cell region, the gate insulating film at the bottom of the gate trenches may break down due to its structure, with application of a high electric field thereto when the MOSFETs are turned OFF. To counter this problem, Patent Document 1 provides, for example, electric field grading regions of a second conductivity type (p-type) at the bottom of the gate trenches (trench-bottom electric field grading regions) to grade the electric field applied to the gate insulating film at the bottom of the gate trenches.
With the structure, extension of a depletion layer from the trench-bottom electric field grading regions to the drift layer enables reduction in the electric field applied to the gate insulating film at the bottom of the gate trenches. The gate trenches inside the cell region further obtain an electric field grading effect from adjacent trench-bottom electric field grading regions at the bottom of the gate trenches. However, gate trenches at the outermost circumference of the cell region cannot obtain the electric field grading effect from outside the cell region because of no trench-bottom electric field grading region formed outside the cell region. Thus, the bottom of the gate trenches at the outermost circumference of the cell region may break because the electric field is concentrated on the region more than the bottom of the gate trenches inside the cell region.
To counter this problem, for example, etching, in an termination region outside a cell region, a drift layer as deep as gate trenches in the cell region to form an outside trench that pierces through a source region and a well region that are stretched from the cell region and thereby form an electric field grading region of a second conductivity type (termination electric field grading region) at the bottom of the outside trench enables grading the concentration of an electric field on gate trenches at the outermost circumference of the cell region and improvement in the withstand performance.